Contents:

Introduction

This group includes clams and mussels which
typically occur in most freshwater habitats and may be particularly
abundant in certain streams. Although the clams and mussels have a wide
range of tolerances to pollution with some species being very sensitive
to water quality, habitat and biological conditions, a number of
species of this group (especially clams) can tolerate somewhat degraded
conditions.

This group is distinguished by:

Characteristic two shells attached by an external hinge which enclose the body of the bivalve.

No eyes or distinct head.

Soft, fleshy body (foot) may be seen extending from shell.

As a general rule, mussels are large and
have a flat, more oblong shell shape, while freshwater clams are
smaller (3/4 inch) and typically more round. In addition, freshwater
clams are usually symmetrical with the umbo (the highest point on the
shell) equally distant from both ends. Mussel shells are usually
lopsided with the umbo closer to the end.

All freshwater clams are filter feeders,
subsisting on phytoplankton, zooplankton, detritus, and bacteria. Some
species of fish consume clams regularly, and several species of
mammals, most notably muskrats and racoons prey heavily on unionaceans.
The shells of unionaceans are used in the Japanese cultured-pearl
industry.

The corbiculid clams (superfamily Sphaeracea, family Corbiculidae) are represented in North America by a single species, Corbicula fluminea. A hermaphrodite, C. fluminea
lives two to three years. The species is said to be intolerant of low
winter temperatures, so it is not clear how widely it will be able to
establish itself in the northeastern United States. C. fluminea
often occurs at spectacularly high density (more than 10,000
individuals/sq.m.). It has been accused of competitively displacing
native bivalves and can cause serious economic problems by attaching to
the inside of and clogging pipes that carry cooling water to power
plants.

The sphaeriid clams
(superfamily Sphaeracea, family Sphaeriidae) are tiny (3-20 mm)
bivalves known as fingernail clams or pea clams. These are
hermaphrodites as well. Sphaeriids live for a year or two. These clams
are especially abundant in standing waters, both permanent and
temporary; a few species are common in running waters.

The two families of clams
of the superfamily Unionacea that occur in the northeastern United
States, the Unionidae and Margaritiferidae, have similar biological
characteristics. The unionaceans are the large (3-20 cm) pearly mussels
of lakes, streams, and rivers. Many species live for 20 years or more,
and one species, Margaritifera margaritifera, has been reported to live for more than a century.

Indicator value

There are two families of bivalves native to North America, the Sphaeriidae (fingernail clams) and the Unionidae (freshwater pearly mussels), and two families that were introduced from Europe, the Corbiculidae (Asian clams) and the Dreissenidae (zebra and quagga mussels). All bivalves are filter-feeding organisms.

The Asian clam is a warm water species
and cannot survive waters that freeze. They are common in enriched
waters and can tolerate water with as little as 50% oxygen saturation,
but not for prolonged periods.

Fingernail clams are closely related to the Asian clams. There are four genera, but only three, Sphaerium, Musculium and Pisidium, are common. Most species of Musculium can be found in temporary aquatic habitats. Most Sphaerium species are large, about 8 to 20 mm; Pisidium species are the smallest, most ranging in shell length from about 2 to 6 mm; and most Musculium species are intermediate in size, about 8 to 10 mm in shell length, and the shells are thin and fragile.

One species, Musculium transversum, is an
enrichment indicator, reaching its largest densities in organically
enriched waters that may have as little as 25% oxygen saturation.

Most other fingernail clams require clean water with
high oxygen tensions. In fact, some fingernail clams are oligotrophic
indicators. While most fingernail clams are not assigned to any
indicator group, they seem to be most abundant in sandy bottoms and
waters with atleast 75% oxygen saturation.

Sphaerium nitidum and Pisidium conventus
reach their greatest densities in the profundal zones of oligotrophic
lakes or in the shallow waters of lakes in high northern latitudes.

Some, like Sphaerium simile, Sphaerium striatinum, Pisidium casertanum, Pisidium compressum, and Pisidium adamsi
are abundant in organic sediments but the waters are usually well
saturated with oxygen, as in many river and stream environments.

The most familiar bivalves are the freshwater pearly
mussels. Most species are large (30-150 mm), but some may grow to
nearly 250 mm in shell length.

Zebra mussels were first discovered in 1988 in Lake
St. Clair but probably first arrived in 1985. Quagga mussels were first
discovered in 1990 in Lake Ontario but probably first arrived in 1988
or 1989.

Because of the quagga mussels' ability to reproduce in
cooler waters and survive in soft substrates, they will be found in
deeper, colder waters of deep lakes and occur further north than zebra
mussels.

Conversely, zebra mussels will probably prevail on
hard substrates in the shallow waters of lakes and will be the main
species in the southern United States where water temperatures are
warmer than found at higher latitudes.

But both species will cause the same kinds of
problems. Because zebra mussels are so prolific in numbers and are so
efficient at filtering the water, there has been a noticeable increase
in the clarity of water in the Great Lakes since their arrival in 1985.
For example, the Secchi depth in Lake Erie had increased from about 1.5
m to about 3.5 m in the eight years that the mussels have been in the
Great Lakes. The water clarity is suspected to have a profound impact
on larval species of fish that feed upon the plankton. This includes
several zooplankton species, larval species of fish that feed upon the
zooplankton, and planktivorous adult fish.

This group includes clams and mussels
which typically occur in most freshwater habitats and may be
particularly abundant in certain streams. Although the clams and
mussels have a wide range of tolerances to pollution with some species
being very sensitive to water quality, habitat and biological
conditions, a number of species of this group (especially clams) can
tolerate somewhat degraded conditions (Peckarskyet al, 1990).

Mussels have larval stages
that are parasitic on specific fish species and are dependent on this
host fish species for dispersal within aquatic systems. As a result,
problems such as barriers to fish movement, or the reactions of mussels
or host fish species to environmental conditions may cause complex and
variable responses in mussel populations. Because of their long life
span and sensitivity to environmental change, most species of mussels
are good indicators of water quality.

"Dead" clams or mussels
(empty shells) do not accurately reflect water quality because shells
can persist for long periods regardless of water conditions. The life
is long, 1 to 15 years in clams, and productivity is relatively low (Kellogg, 1994).